电化学还原法制备金属钛的阴极烧结工艺研究

阴极制备是用电化学还原法从中提取金属钛工艺的关键步骤.TiO2阴极制备包括成形和烧结过程.采用SEM、EDS等方法研究了烧结温度、烧结时间对TiO2阴极微观结构、孔隙率的影响及不同烧结条件下TiO2阴极电解产物结构和含氧量的变化.烧结条件主要影响到TiO2阴极的颗粒尺寸、孔隙率和孔隙尺寸,而这3个因素同时影响电脱氧过程.在1 000～1 100 ℃下烧结4 h获得的TiO2,阴极脱氧效果较好.     

电化学还原TiO2制备金属钛及反应过程的研究

采用熔盐电解法,在900℃熔盐CaCl2中以烧结TiO2为阴极,石墨棒为阳极制备出了金属钛.研究了不同温度下烧结阴极的形貌及其对电解反应的影响.结果表明：600℃下烧结4 h的阴极具有良好电化学反应性能;电解过程中脱氧速度不均匀;TiO2电极的还原是由外向内,由高价向低价再到金属分步进行的;电解还原过程电流效率低约为15%.阴极的孔隙大小和颗粒尺寸会影响电极的反应速度.     

电脱氧法由Nb2O5直接制备金属铌

采用熔盐电脱氧法,在800℃混合熔盐CaCl2-NaCl中以烧结的Nb2O5为阴极,石墨坩锅为阳极制备了金属铌.研究了不同温度下烧结阴极形貌及其对电脱氧反应的影响,并考查了电解时间对阴极产物中氧含量的影响.结果表明:1 200℃下烧结12 h的阴极片具有良好的电化学反应性能;电脱氧过程中阴极片的脱氧速度不均匀;Nb2O5电极的还原由外向内,由高价向低价分步进行;阴极的孔隙大小和颗粒尺寸会影响电极反应的速度.     

TiO2电化学还原工艺中阴极制备方法研究

将TiO2粉末烧结成阴极片后直接在CaCb融盐中进行电化学还原制取钛的工艺是目前的研究热点。在粉末成型过程中用碳粉代替粘接剂，研究了成形压力、碳粉用量及水的用量对成型后阴极片孔隙率的影响；在3．1V电压下，950℃的CaCl2融盐中进行电化学还原实验，借助SEM，EDS及XRD分析了电解产物的形貌、成分及物相；分析了阴极孔隙对电解过程的影响。结果表明：加入碳粉，减小成形压力可以增大TiO2阴极的孔隙率，水的用量对阴极孔隙率影响不大；添加碳粉的TiO2阴极可以还原成海绵钛。     

三相界面反应机制在SOM法制备金属钽中的应用

利用同体透氧膜(SOM)法电解还原Ta2O5制备金属钽,通过三相界面反应机制讨论了阴极微结构对电解过程及产物形貌的影响.结果表明:阴极片的颗粒尺寸与孔隙率是影响有效三相界线的重要因素;孔隙率大和粒度小均有利于电解还原的进行,孔隙率小会导致阴极产物形成致密的金属钽外层,阻碍阴极进一步脱氧;成型压力4 MPa时1150℃烧结2 h制备的阴极片具有合适的孔隙率和拉度,电化学活性良好,电解产物形貌均匀,氧含量低.     

阴极结构对SOM法制备金属钽的影响

在CaCl2熔盐中,利用固体氧离子膜(SOM)法电解还原Ta2O5直接制备金属钽。研究了成形压力与烧结温度对阴极形貌及孔隙率的影响,进而获得阴极微结构对产物形貌及氧含量的影响规律。结果表明:阴极片的颗粒尺寸和孔隙率大小是影响电解还原的重要因素;Ta2O5片的孔隙率随着成形压力的增大、烧结温度的升高明显降低;孔隙率大,有利于电解还原的进行,产物颗粒尺寸大;孔隙率小,会导致阴极产物形成致密的金属外层;在成形压力为4MPa经1150℃烧结2h或6MPa经1100℃烧结2h条件下,阴极反应活性良好,电解产物氧含量低。     

TiO2阴极组分对电解的影响

在熔盐电解制备钛的TiO2固体阴极中分别添加CaO、CaCO3粉末,研究经烧结后阴极的形貌及成分,考察添加不同组分对电解提取钛反应的影响.结果表明在TiO2阴极中添加CaO、CaCO3粉末均可以增加烧结后阴极片的孔隙、影响TiO2颗粒尺寸的大小,有利于加快阴极电解反应的速度,并研究电化学还原机理.     

氩气保护烧结多孔TiNi合金孔隙特征的研究

采用氩气保护烧结制备了多孔TiNi合金，并用图像分析的方法对其孔隙和孔隙率进行了研究。结果表明：粉末粒度和压制压力影响压坯中的孔隙和粉末颗粒之间的结合，从而影响烧结产物的孔隙；烧结温度和烧结时间是影响多孔TiNi合金孔隙的重要因素；980℃烧结时孔隙形成发展过程分为三个阶段：新孔形成、孔隙的聚集和孔隙的收缩，烧结8h所得到的多孔TiNi合金的孔隙率较高；1000℃下烧结0．5h孔隙已经基本形成，延长时间孔隙率变化不大。     

CaCl2-NaCl熔盐电脱氧法制备金属Ta

在800℃的CaCl2-NaCl混合熔盐体系内，以烧结后的Ta2O5片体做阴极、高密度石墨碳棒做阳极，在工作电压为3．1V的条件下进行电解脱氧反应．研究了1000℃烧结4h前、后Ta2O5片体的孔隙率和微观形貌，以及阴极片结构对电解还原反应过程的影响；利用XRD分析了电解产物的相组成，采用称重法测量了电解产物的氧含量．结果表明，采用熔盐电脱氧法可以制备出金属Ta，其阴极片的大孔隙率有利于电解脱氧反应的进行；扩散过程是影响熔盐电脱氧反应速度的重要环节．     

SOEC阴极材料Sr_2FeMoO_(6-δ)的微观结构和性能

选用双钙钛矿结构Sr_2FeMoO6-δ作为固体氧化物电解池的阴极材料,经过压制成型和烧结制备成阴极。利用阿基米德法测定了阴极的孔隙率,结合扫描电子显微镜研究造孔剂的用量对阴极孔隙结构的影响。利用热分析仪测定了不同孔隙结构的阴极受热后热膨胀情况和热膨胀系数,研究其与电解质的热膨胀系数匹配情况。最后利用电化学工作站测试了阴极材料的电化学性能。实验结果表明,双钙钛矿结构Sr2Fe Mo O6-δ有较好地电化学性能以及与电解质LSGM热膨胀系数匹配,有望成为固体氧化物电解池阴极的理想候选材料。     

The sintering behavior of quasi-spherical tungsten nanopowders

In this work, the sintering behavior of quasi-spherical tungsten nanoparticles was investigated by analysis the sintered compacts obtained at different sintering temperatures and dwell time, and the influence of microstructures on the density and Vickers microhardness of sintered products was also studied. Experimental results show that particle shape and size distribution are critical to the sintering activity and mechanical properties of obtained compacts. 91.3% of theoretical density (TD) of the compact could be obtained at low sintering temperature of 1500 °C, and the highest hardness of 606 VHN could be achieved when sintered at 1100 °C due to formation of uniform, densely packed sintered compacts with grain size of 235.7 nm. Importantly, unusual linear correlation between grain size and relative density was observed in our experiment, and a cut-off point exists at 85.6% of TD. The kinetic analysis revealed that surface diffusion is responsible for the mass transport during the initial sintering stage.     

Sintering process and grain growth of Mn-Zn ferrite nanoparticles

The density, microstructure and magnetic properties of non-doped Mn-Zn ferrite nanoparticles sintered compacts were investigated. The compacts of non-doped Mn-Zn ferrite nanoparticles were sintered by segmented-sintering process at lower sintering temperature. The density of sintered samples was measured by Archimedes method, and the phase composition and microstructure were examined by XRD and SEM. The sintered Mn-Zn ferrite magnetic measurements were carried out with Vibrating Sample. The results show that the density of sintered compacts increases with the rising of sintering temperature, achieving 4.8245 g·cm-3 when sintered at 900 ℃, which is the optimal density of Mn-Zn functional ferrite needed and from the fractured surface of sintered samples, it can be seen that the grain grows well with small grain size and homogeneous distribution.     

Improving the uniformity in mechanical properties of a sintered Ti compact using a trace amount of internal lubricant

Large sintered powder compacts are likely to be associated with variability in mechanical properties; an improvement of the uniformity of the mechanical properties of sintered powder compacts is important for powder metallurgy. In this work 0.3–1 wt.% stearic acid (SA) or magnesium stearate (MgSt) was added to a 40 mm diameter Ti powder compacts with height to depth (H/D) ratio of unity to give a more uniform green density. Tensile test pieces were cut from selected positions in each sintered compact to obtain the distribution of mechanical properties. Results revealed that variations in mechanical properties are due to the pore morphology with respect to size, aspect ratio and preferred orientation. A trace amount of lubricant significantly improves the uniformity in mechanical properties by optimizing the porosity distribution and minimizing the pore size and aspect ratio of pores after sintering. Such an effect was achieved by reducing the initial green density inhomogeneity and the stress induced by the mismatch of sintering shrinkage. However a relatively high 1 wt.% SA addition with a large particle size created burnt-off pores in the top and bottom zones. MgSt is not recommended since it significantly increases the oxygen content. An addition of 0.6 wt.% SA is the best choice due to the even pore distribution, small pore size and acceptable level of oxygen pick up.     

植物种子模板法制备Al_2O_3泡沫陶瓷的研究

将小米种子排列成模板,通过在模板内对Al2O3浆料进行离心成形来制备孔径均匀的Al2O3泡沫陶瓷。观察了浆料固相含量对Al2O3生坯密度均匀性的影响,研究了植物种子特性对Al2O3生坯干燥和焙烧行为的影响,测量烧结产物的密度、收缩率、孔隙率和压缩强度,并通过体式显微镜和扫描电镜观察Al2O3泡沫陶瓷的宏观结构和显微组织。研究结果表明,小米和Al2O3生坯具有较好的收缩匹配性,干燥后可以保持生坯形状完整。在较高的固相含量(50%,体积分数)下,离心时的物质分离现象被抑制,孔壁具有较高的生坯相对密度(63.8%)。1500℃烧结2h后,Al2O3泡沫陶瓷具有致密的孔壁和较高的烧结相对密度(98.9%),其孔隙度为66.5%,抗压强度为5.26MPa。     

高密度纯钨的低温活化烧结工艺及其致密化行为

采用溶胶喷雾干燥-煅烧-氢热还原法制备了BET粒径为0.21μm的超细纯钨粉末,并利用球磨处理进一步活化粉末。研究了超细纯钨粉末形貌及其性能随球磨时间的变化特征,探索了未球磨、球磨5h及球磨10h3种超细纯钨粉末烧结致密工艺,此外还详细研究了纯钨烧结体组织形貌、晶粒尺寸及显微硬度等性能随烧结温度及球磨时间的变化规律。结果表明,球磨处理对超细纯钨粉末的烧结起到了极大的活化作用,由球磨10h粉末制成的压块在1900℃下烧结2h其致密度即可达97.3%,比传统微米级纯钨粉末制成的压块达到相同烧结致密度的温度降低了600℃以上。同时,球磨处理可以大幅降低钨粉的起始烧结温度和再结晶温度,获得组织更加均匀细小、力学性能(硬度)更加优良的钨烧结体。     

Microstructural evolution during sintering of near-monosized agglomerate-free submicron alumina powder compacts

Colloidally processed near-monosized, agglomerate-free submicron alumina powder compacts were sintered under different conditions to study the evolution of pore structures during sintering. The results showed that even though the compacts were agglomerate-free to start with, agglomeration took place during sintering due to local densification of the particles with different co-ordination numbers. Inter-agglomerate channel-like pores were formed as a result, which eventually evolved into an isolated pore on further sintering. The densification rate was controlled by mass transport via grain boundary diffusion before the formation of isolated inter-agglomerate pores, after which it was controlled by the sinterability of the pores. From this stage on, grain growth was required to bring about further sintering. At low sintering temperatures, grain growth was sluggish, probably a result of impurity controlled grain growth. This resulted in an abrupt drop in the densification rate and the phenomenon of end density at low sintering temperatures. The present work shows that an initial agglomerate-free green structure of fine, monosized particles is essential to resist particle agglomeration and grain growth during sintering, so as to achieve a low sintering temperature and a fine grain size sintered product.     

溶胶-喷雾干燥超细/纳米晶W-20Cu复合粉末的烧结行为

采用喷雾干燥-氢气还原法制备超细/纳米晶W-20Cu(质量分数,%)复合粉末,粉末压坯直接从室温推入高温区烧结不同时间后直接取出水淬,研究其烧结致密化和显微组织的变化。结果表明,超细/纳米晶W-20Cu粉末在1000～1200℃烧结时发生迅速致密化。粉末压坯在1200℃烧结60min,其材料致密度已达到96.4%。1420℃烧结90min时致密度达到99%以上。1100～1420℃烧结时其烧结致密化活化能不断减小,从1100℃时的276.3kJ/mol减小到1420℃时的29.1kJ/mol。当温度低于1200℃时,W晶粒长大不明显,当温度超过1300℃时,W晶粒开始有明显长大。随温度的升高W晶粒发生显著球形化,1420℃烧结时发现其晶粒长大符合G3=kt的Ostwald机制,此时晶粒长大动力学系数K仅为0.024μm3/min。     

Effect of heating mode and sintering temperature on the consolidation of 90W-7Ni-3Fe alloys

The present study compares the sintering response of 90W-7Ni-3Fe alloys consolidated in a 2.45 GHz microwave furnace and a conventional furnace. The W-Ni-Fe compacts were sintered in a temperature range of 1200-1500 °C corresponding to solid-state as well as liquid phase sintering. The compacts were successfully sintered in a microwave furnace with about 80% reduction in the overall processing time. For both the heating modes, the W-Ni-Fe alloys exhibited significant densification prior to melt formation through solid-state sintering. The in situ dilatometric studies revealed that the contribution to densification from solid-state sintering is higher at lower heating rates. In comparison to conventional sintering, microwave sintered compacts showed relatively refined microstructure and higher hardness and flexural strength.     

Preparation of Ti by direct electrochemical reduction of solid TiO2 and its reaction mechanism

An electrochemical method for the direct reduction of solid TiO2 was introduced, in which the oxygen is ionized, dissolved in the molten salt and discharged at the anode, leaving pure titanium at the cathode. Titanium was prepared from TiO2 via molten salt electrolysis, and experimental process was studied in detail. The reductive products of TiO2 were analyzed through XRD, SEM, XPS and so on. The results indicate that titanium with high purity is obtained by the electrochemical reduction. Through thermogravimetric analysis of molten CaCl2, it is shown that the dehydration of CaCl2 has to be strictly controlled to eliminate hydrolysis. The cyclic voltammetry studies on the TiO2 cathode electrochemical system show that the reduction of TiO2 is conducted step by step, from exterior to interior and from high-valence to low-valence. The conclusions were also confirmed by study on thermodynamics analysis. The current efficiency can be improved by loading different voltages in different stages.